1. Field of the Invention
The present invention pertains to a method and system for reducing hydrocarbon vapor emissions from crude oil tankers and the like by treatment of an inert gas system for filling the tank spaces during transit and loading procedures.
2. Background
In conventional operations for loading crude oil and refined petroleum products tankers and the like, a considerable amount of hydrocarbon vapor may be generated during the loading process due to the temperature difference between the liquid entering the storage tanks and the ambient temperature of the vapor or inert gas occupying the tanks during the loading process. In many locations the temperature of the oil being loaded greatly exceeds the temperature of the gas or vapor in the tank at the time of loading. Considerable vapor generation occurs because the gas or vapor being emitted from the liquid oil near the oil-gas interface is heated and thus becomes less dense than the overlying gas. This buoyancy causes an interchange thereby lifting the evaporated hydrocarbon vapors higher into the vapor space and returning the inert gas to the vicinity of the oil-gas interface. This mechanism tends to reduce the partial pressure of hydrocarbon vapors near the oil-gas interface and thereby accelerates the rate of vapor diffusion away from the interface. Accordingly, further evaporation of hydrocarbons is accelerated. The present invention pertains to a method and system for reducing the amount of hydrocarbon vapor emissions when loading tankers with crude oil or other petroleum liquids having a temperature greater than the ambient temperature. The method and system may be applied to tankers as well as other storage facilities for storing crude oil and other petroleum liquids.